Power converter circuitry is used in a variety of applications to convert and/or condition power from an input source in order to provide a desired output voltage and output current. While there are many different types of power converter circuitry, one type that is currently in widespread use is the switching power converter. Switching power converters include at least one switch, which is used to selectively deliver power from an input source to one or more additional components therein to provide a desired output voltage and output current. Control circuitry for a switching power converter provides control signals to the switch to change the output voltage, the output current, or both.
A conventional switching power converter 10 is shown in FIG. 1 for purposes of illustration. The conventional switching power converter 10 is a single-ended primary-inductor converter (SEPIC) including a transformer 12, a switch 14, a diode 16, and a capacitor 18. The transformer 12 includes a primary winding 20A coupled between an input node 22 and the switch 14 and a secondary winding 20B coupled between an anode of the diode 16 and ground. The switch 14 is coupled between the primary winding 20A and ground. The cathode of the diode 16 is coupled to an output node 24. The capacitor 18 is coupled between the output node 24 and ground.
Power converter control circuitry 26 is coupled to the switch 14, and may also be coupled to the input node 22 and the output node 24 in order to receive feedforward and feedback signals therefrom. In operation, the power converter control circuitry 26 provides a switching control signal SC to the switch 14. The switch 14 may be a transistor such as a field-effect transistor (FET) including a gate (G) a drain (D), and a source (S). The switching control signal SC may be provided to the gate (G) of the switch 14 in order to control the amount of current flowing from the drain (D) to the source (S) thereof. When the switch 14 is closed (ON), a primary winding current IPW flows through the primary winding 20A. The current through the primary winding 20A induces a reverse current in the secondary winding 20B. This reverse current is blocked by the diode 16, and energy therefore accumulates in a magnetic field of the transformer 12. When the switch 14 is open (OFF), current does not flow through the primary winding 20A, and the magnetic field of the transformer 12 collapses such that a secondary winding current ISW is provided through the diode 16 to the output node 24. This secondary winding current ISW charges the capacitor 18, which sources voltage and current to the output node 24 both when the switch 14 is open and closed in order to reduce ripple in the output voltage and/or output current.
Switching power converters can be operated in several different modes. In a continuous conduction mode (CCM), the switch control signal SC is provided such that when the switch 14 is opened, the secondary winding current ISW does not fall to zero before the switch 14 is closed again. In a discontinuous conduction mode (DCM), the switch control signal SC is provided to the switch 14 such that when the switch 14 is opened, the secondary winding current ISW falls to and stays at zero for some period of time (i.e., “dead time”) before the switch 14 is closed again. In a boundary conduction mode (BCM), the switch control signal SC is provided such that when the switch 14 is opened and the secondary winding current ISW reaches zero, the switch 14 is immediately closed again.
Waveforms illustrating the continuous conduction mode (CCM), the discontinuous conduction mode (DCM), and the boundary conduction mode (BCM) are shown in FIGS. 2A-2C, respectively. In these figures, the switch control signal SC is shown along with the primary winding current IPW and the secondary winding current ISW. When the switch control signal SC is high, the switch 14 is closed. When the switch control signal SC is low, the switch 14 is open. As discussed herein, the amount of time the switch control signal SC is high and thus the amount of time the switch 14 is closed is the on time TSWON of the switch 14. The amount of time the switch control signal SC is low and thus the amount of time the switch 14 is open is the off time TSWOFF of the switch 14. The combined on time TSWON and off time TSWOFF of the switch 14 defines a switching period PSW.
Conventionally, power converter control circuitry has been designed using analog components. The resulting analog control circuitry was often complex and consumed a large amount of space. Recently, there has been a trend towards digital control circuitry for switched power converters. Digital control circuitry such as a microcontroller can provide the switch control signal SC using pulse-width modulation (PWM). While using digital control circuitry in this manner may result in reduced complexity and saved space, there are several issues with doing so, especially when the digital control circuitry is to be used with a switching power converter for a light-emitting diode (LED) light.
Generally, it is desirable for LED lights to be dimmable across a large range of values (e.g., from 5% to 100% of their brightness capability). In order to provide this dimming capability, a switching power converter used in an LED lighting application must be capable of supplying output voltage and/or output current across a relatively large range of values. When operated in a boundary conduction mode (BCM) as discussed above, the power converter control circuitry 26 provides the switch control signal SC to adjust the on time of the switch 14, which in turn provides a desired output voltage and/or output current from the conventional switching power converter 10. The off time of the switch 14 is set implicitly, as it is controlled by the amount of time it takes for the secondary winding current ISW to reach zero after the switch 14 is turned off. In such an approach, the on time of the switch 14 may become very small when attempting to provide a low output voltage and/or output current in order to achieve desired dimming of an LED light. This small on time necessitates a high switching speed and thus a high frequency of operation of PWM circuitry that may be used to provide the switching control signal SC in digital control circuitry. Further, such high-frequency switching demands high-resolution PWM circuitry in order to set the on time of the switch 14 with a desired amount of precision. The resolution of the PWM circuitry becomes increasingly important as the switching frequency increases. Often, the cost and complexity of digital controllers such as microcontrollers increases in proportion to the speed and resolution of PWM circuitry therein. Since switching power converters for LED lights may require relatively high switching frequencies as discussed above, using digital control circuitry therewith may be cost prohibitive.
One way to solve the aforementioned problems is by operating the conventional switching power converter 10 in a discontinuous conduction mode (DCM) as discussed above. Accordingly, the power converter control circuitry 26 provides the switch control signal SC such that the on time of the switch 14 remains constant while the off time thereof is adjusted. Such a control scheme allows the conventional switching power converter 14 to provide a very low output voltage and/or output current and thus achieve a desired level of dimming for an LED light. However, the switching frequency risks becoming too low in a discontinuous conduction mode (DCM) at which point the circuitry may produce undesirable audible noise.
Another way to solve the aforementioned problems is using frequency limiting, in which the switching frequency of the switch 14 is clamped on the high end, the low end, or both. Such frequency limiting is often used along with analog controllers. However, clamping the switching frequency of the switch 14 limits the range of output voltage and/or output current of the conventional switching power converter 10 and thus the dimming capability of an LED light used therewith. In addition to frequency limiting, burst operating modes may also be used wherein entire switching periods are skipped in order to provide the output voltage and/or output current at a desired level. However, burst operating modes may also introduce audible noise and/or ripple in the output voltage and/or output current.
In light of the above, there is a need for power converter circuitry with an improved control scheme such that the power converter circuitry can be used with digital control circuitry while providing a large output voltage and/or output current range suitable for LED lighting applications.